1. Field of the Invention
The present invention relates to an image forming apparatus that applies heat and pressure to a recording medium having an image formed thereon to fix the image.
2. Description of the Related Art
Conventionally, in an image forming apparatus that forms an image on a recording medium in accordance with the electrophotographic system, a fixing device is used in order to fix a toner image formed by a toner consisting of thermo-melting type resin or the like on the recording medium. In a sublimation type or thermal transfer type printer and the like, a fixing device is also used for fixing of an image formed on a recording medium and modification of surface properties of the recording medium.
Most of fixing devices adopted in electrophotographic apparatuses and the like are heat-pressing type fixing devices that heat and press recording mediums to melt and fix toner images placed thereon.
The heat-pressing fixing devices are roughly categorized into two types. One is a fixing device of a roller type fixing system in which a pair of rollers are opposed to be in press-contact with each other, a heating source is arranged in one or both of the rollers, and a recording medium is conveyed while being nipped in a press-contact section of the rollers to perform fixing process. The other is a fixing device of a so-called belt type fixing system in which one of pressing members is constituted by a roller and the other is constituted by a belt.
The roller type fixing system has, as a basic structure, a roller pair of a fixing heat roller and a pressure roller in which temperature of the fixing heat roller is adjusted to a predetermined fixing temperature, and the pressure roller comes into press-contact with the fixing roller. This roller pair is rotated to lead a recording medium having an unfixed toner image formed thereon into a fixing nip section, which is a press-contact section of the roller pair, to nip and convey the recording medium. The unfixed toner image is heat-pressed and fixed on the recording medium by the heat of the fixing roller and the pressure at the fixing nip section.
A heat quantity given to the recording medium having the unfixed toner image deposited thereon depends on the temperature of the fixing roller and the pressure roller and time during which the recording medium passes the fixing nip section, that is, a fixing nip width and conveying speed of the recording medium. The fixing nip width indicates length of the recording medium in a conveying direction at the fixing nip section.
As a fixing device mounted on an electrophotographic apparatus or the like having a higher process speed, a fixing device with a larger fixing nip width is required because of the relation with the heat quantity. An increase in diameters of the rollers is required in order to increase the fixing nip width in the roller type fixing system. However, when the diameters of the rollers are increased, there are problems in that, for example, a heat capacity of the rollers increases and accordingly, heat-up time (warm-up time) of the rollers increases.
As a fixing device structure with which a large fixing nip width can be secured without increasing diameters of rollers as disclosed in, for example, Japanese Patent Laid-Open No. 61-132972, wherein a fixing device of a belt type fixing system is proposed.
In such a fixing device of the belt type fixing system, a heat-resistant and flexible endless belt stretched among plural belt stretching members is brought into press-contact with a fixing roller serving as a rotating member for heating to form a fixing nip portion. A recording medium having an unfixed toner image born thereon is nipped and conveyed by this fixing nip section. Consequently, the unfixed toner image is heat-pressed and fixed on the recording medium by the heat of the fixing roller and the pressure of the fixing nip section. In this fixing device of the belt system, it is possible to easily set a fixing nip width larger according to adjustment of a contact width of the endless belt against the fixing roller. Since it is possible to secure the fixing nip width without depending on the diameter of the fixing roller, it is possible to form the fixing roller to have a small diameter with a small heat capacity and to reduce a warm-up time.
In order to stabilize the quality of an image finally obtained in an electrophotographic apparatus or the like, control of heat applied to an image forming surface is important. Therefore, various systems are proposed for temperature control or temperature adjustment for a fixing device. For example, in Japanese Patent Laid-Open No. 2004-163868, a fixing device having a structure including heaters in upper and lower pressing members is disclosed. In this Japanese Patent Application Laid-Open No. 2004-163868, it is mentioned that maximum electric power is supplied to an upper heater included in the upper pressing member and, when the temperature of the fixing device reaches a predetermined temperature, electric power supplied to the upper heater is reduced and electric power is also supplied to a lower heater included in the lower pressing member to perform the temperature control.
More specifically, at the time of a warm-up operation, in a state in which a fixing belt and an opposed roller are brought into contact with each other, maximum electric power is supplied to first heating means provided on the inside of a heating roller of the fixing belt to heat the fixing belt only with the first heating means. Subsequently, when the temperature of the fixing belt reaches a first reference temperature, the fixing belt is rotated, and simultaneously the supplied electric power to the first heating means is reduced. Moreover, electric power is also supplied to second heating means provided on the inside of the opposed roller to make it possible to perform a fixing operation when the temperature of the fixing belt reaches a target temperature and the temperature of the opposed roller reaches a second reference temperature.